Compared with single-crystalline silicon solar cells and multi-crystalline silicon solar cells mainly used as solar cells, thin film silicon solar cells attract attention as solar cells of the next generation since they are advantageous in view of cost without using expensive silicon substrates.
As a method of manufacturing an amorphous silicon thin film used for thin film silicon solar cells, a manufacturing method using a parallel-plates type plasma CVD device is known. A conventional parallel-plates type plasma CVD device used in this manufacturing method is shown in FIG. 7.
The conventional parallel-plates type plasma CVD device 61 shown in FIG. 7 has a vacuum vessel 62 for performing plasma treatment. The vacuum vessel 62 usually has exhaust ports 62a to be connected with a high-vacuum exhaust equipment and a process exhaust equipment. The high-vacuum exhaust equipment is used to obtain the back pressure inside the vacuum vessel 62, and as the high-vacuum exhaust equipment, usually a turbo molecular pump or the like is used. The process exhaust equipment is used to maintain the pressure required in a plasma treatment process, and in the case of a general CVD process, as the process exhaust equipment, a mechanical booster pump or the like is used, though depending on the process pressure.
Inside the vacuum vessel 62, a discharge electrode plate 63 and an earth electrode plate 610 are installed to face each other with a clearance therebetween. On the upper surface of the earth electrode plate 610, a substrate 612 is held. The earth electrode plate 610 is internally provided with a heating mechanism 611 for heating the substrate 612.
At the lower face of the discharge electrode plate 63, a hollow portion 63a is provided, and a shower plate 66 is installed at the lower face of the discharge electrode plate 63, to close the hollow portion 63a. In the shower plate 66, numerous gas introduction holes 66a are provided therethrough from the upper surface to the lower surface of the shower plate 66. The vacuum vessel 62 is provided with a raw gas supply pipe 65 extending from a gas supply equipment (not shown in the drawing) installed outside the vacuum vessel 62 and passing through the discharge electrode plate 63, to reach the hollow portion 63a. 
The raw gas supply pipe 65 is electrically insulated from the discharge electrode plate 63 though not shown in the drawing. The vacuum vessel 62 is also electrically insulated from the discharge electrode plate 63 though not shown in the drawing. The vacuum vessel 62 is earthed by a conductor 62c. Between the vacuum vessel 62 and the earth electrode plate 610, an insulator 610a is provided, and the earth electrode plate 610 is earthed by a conductor 610c. 
Raw gas necessary for plasma treatment is supplied from the raw gas supply equipment through the raw gas supply pipe 65 into the hollow portion 63a. The gas supplied into the hollow portion 63a passes through the numerous gas introduction holes 66a of the shower plate 66 and is uniformly supplied to the substrate 612 held on the earth electrode plate 610.
A high frequency power supply 614 is connected with the discharge electrode plate 63 via a matching box 613. The exhaust equipment keeps the inside of the vacuum vessel 62 at a constant pressure, and a high frequency power is applied to the discharge electrode plate 63 by the high frequency power supply 614, to generate plasma. Generated plasma forms an amorphous silicon thin film on the surface of the substrate 612.
However, it is known that if the amorphous silicon thin film produced by using such a parallel-plates type plasma CVD device is irradiated with light, dangling bonds (defects) increase in the film, to cause light-induced degradation. The problem of light-induced degradation was found as the Staeber-Wronski effect more than 30 years ago, but is not yet solved.
The mechanism in which the light-induced degradation is caused is not yet clearly clarified. However, it is known that the light-induced degradation has correlation with a Si—H2 bond concentration in the film. Further, it is reported that if the Si—H2 bond concentration in the film is low, the light-induced degradation is also small. It is indicated as a cause of the increase in the Si—H2 bond concentration that high order silane-related species (SimHn: m=2 or more) growing during formation of the film are incorporated into the film. It is considered that the high order silane-related species grow due to the successive reaction in which the SiH2 radicals produced in the plasma are inserted into Si—H bonds, and are mixed into the film, to increase the Si—H2 bonds, causing initial dangling bonds to be formed in the film.
On the other hand, the reactions in the plasma start when electrons having some energy collide with SiH4 acting as parent molecules, to decompose them into various molecules such as SiH3 radicals and SiH2 radicals. In general, the electron temperatures (Te) showing energy of electrons in the plasma have a distribution, and in addition to the SiH3 radicals as a precursor contributing to creation of the film, SiH2 radicals are produced without fail. For this reason, in the case where the conventional parallel-plates type plasma CVD device is used to manufacture an amorphous silicon thin film, the power applied is set at a low level in order to decrease the generation of high order silane-related species, for thereby inhibiting the generation of SiH2 radicals and high order silane-related species. However, because of the low power level, the depositing rate cannot be enhanced (Non Patent Literature 1).
On the other hand, as a film deposition method for obtaining an amorphous silicon thin film with a low Si—H2 bond concentration, a triode deposition system is proposed. A plasma CVD device using the triode deposition system is shown in FIG. 8. The plasma CVD device 71 using the triode deposition system shown in FIG. 8 is identical to the plasma CVD device 61 shown in FIG. 7 in basic structure. Accordingly the same components as those of FIG. 7 are indicated by the same symbols in FIG. 8. The difference between the device 71 of FIG. 8 and the device 61 of FIG. 7 is that a mesh electrode plate 716 is installed between the discharge electrode plate 63 and the earth electrode plate 610.
In FIG. 8, a DC variable power supply 715 is connected with the mesh electrode plate 716. As can be seen from FIG. 8, the triode deposition system also uses a parallel-plates type CVD device. The mesh electrode plate 716 is inserted between the discharge electrode plate 63 and the earth electrode plate 610, and a potential (usually a negative potential) is applied to the mesh electrode plate 716. Thus, it is considered that the plasma can be contained between the discharge electrode plate 63 and the mesh electrode plate 716. No plasma is generated between the mesh electrode plate 716 and the earth electrode plate 610. On the other hand, the radicals contributing to creation of the film are produced between the discharge electrode plate 63 and the mesh electrode plate 716 and diffused by the mesh electrode plate 716, to reach the substrate 612.
The diffusion distance of radicals is proportional to the square root of the inverse number of the molecular weight. Therefore, it is intended to use that the diffusion distance of high order silane-related radicals is shorter than that of SiH3 radicals, in order to selectively transport the SiH3 radicals to the substrate 612.
With this configuration, a very low Si—H2 bond concentration can be achieved to obtain an amorphous silicon thin film having a low light-induced degradation rate. However, in order to remove high order silane-related radicals in the triode deposition system, it is necessary that the distance between the mesh electrode plate 716 and the earth electrode plate 610 is long enough. For this reason, the triode deposition system has a problem that the depositing rate cannot be enhanced (Non Patent Literature 2).
Further, the gas temperature in the plasma is also an important factor. It is known that the successive reaction for growing high order silane-related species (SimHn: m=2 or more) is a third-body reaction. As a means for inhibiting this reaction, gas heating is considered effective. The high order silane-related species produced by the insertion reaction of SiH2 radicals into Si—H bonds are stabilized by making a third body (usually SiH4 acting as parent molecules) absorb extra energy.
Consequently in the state where the third body is not in a position to accept energy, that is, in the state where the temperature is high, the third-body reaction does not take place, and high order silane-related species are inhibited (Patent Literature 1). Therefore, in order to heat the space for depositing the film, it is desirable that the plasma near the sheath on the discharge electrode plate side where the high order silane-related species are considered to be most generated is heated from the discharge electrode plate side. However, it is structurally difficult to apply a high frequency to the electrode plate and further to introduce a heater. Usually in order to control the substrate temperature, the earth electrode plate supporting the substrate is heated. This causes also the plasma to be heated via the substrate, but since the place is distant from the sheath on the discharge electrode plate side, the state of effective and positive heating cannot be achieved. Accordingly if the substrate temperature is further raised to higher than the optimum substrate temperature, defects in the film increase. Therefore, there is a problem that the highest heating temperature is limited.